In an apparatus described to JP-2001-82221A, an air-fuel ratio sensor (A/F sensor) which detects an air-fuel ratio of the exhaust gas is installed at an exhaust confluent part into which the exhaust gas of a plurality of cylinder flows, and the catalyst for exhaust-gas clarification is installed downstream of the air-fuel ratio sensor. A signal of the air-fuel ratio sensor is extracted for each cylinder, and fuel injection quantity is controlled with respect to each cylinder based on the extracted signal. Thereby, the air-fuel-ratio dispersion between the cylinders is corrected for each cylinder, and the exhaust air purification of the catalyst is enhanced.
As shown in FIG. 4, in the output characteristics of the air-fuel ratio sensor, there is a region in which the output current (limiting current value) changes almost linearly according to the air-fuel ratio λ. The output current of this air-fuel ratio sensor is detected, and it is changed into the air-fuel ratio λ. The output characteristics of this air-fuel ratio sensor varies due to a manufacturing tolerances, a deterioration with age, and the like. The inclination of the output-characteristics line changes due to the manufacturing tolerances, the deterioration with age, and the like. For this reason, even if an air-fuel ratio control for each cylinder is performed, the dispersion in output characteristics deteriorates the detecting accuracy of the air-fuel ratio and the accuracy of the air-fuel ratio control.